The compound class of the π-conjugated polymers has been the subject of many publications in the past few decades. They are also called conductive polymers or synthetic metals.
Conductive polymers are increasing in economic importance, as polymers have advantages over metals with regard to processability, weight and the targeted setting of properties by chemical modification. Examples of known π-conjugated polymers are polypyrrols, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes).
A particularly important polythiophene used industrially is poly-3,4-(ethylene-1,2-dioxy)thiophene, often also called poly(3,4-ethylene dioxythiophene), which has very high conductivity in its oxidised state and is disclosed for example in EP-A 339 340 or EP-A 440 957. A summary of numerous poly(alkylene dioxythiophene) derivatives, in particular poly-(3,4-ethylene dioxythiophene) derivatives, their momomer structural elements, syntheses and applications is given in L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 2000, 12, p. 481-494.
The production of highly-conductive poly(3,4-ethylene dioxythiophene) derivatives from 2,5-dihalogen-3,4-ethylene dioxythiophenes has been disclosed only recently. According to the method disclosed by Wudl et al. the 2,5-dihalogen-3,4-ethylene dioxythiophenes are converted in a solid phase reaction to the halogen-doped polythiophenes (H. Meng, D. F. Perepichka and F. Wudl, Angew. Chem. 2003, 115(6), p. 682-685 and H. Meng, D. F. Perepichka, M. Bendikov, F. Wudl, G. Z. Pan, W. Yu, W. Dong and S. Brown, J. Am. Chem. Soc. 2003, 125, p. 15151-15162). Although this process produces the desired highly-conductive polymers, it does have some disadvantages. On the one hand, the solid phase synthesis produces products the conductivity of which is difficult to set and which are highly-dependent on reaction conditions (time, temperature). The maximum conductivity is achieved only after a lengthy reaction time (Angew. Chem.). Furthermore (J. Am. Chem. Soc.) the doped polythiophenes are not temperature-stable and give off highly-toxic halogen e.g. Br at just above room temperature. This behaviour is not compatible with an industrial application. By subsequently treating the 2,5-dibromo-3,4-ethylene dioxythiophene with concentrated strong acids (sulfuric acid, trifluoromethane-sulfonic acid) a material with highly-reduced conductivity is obtained with the release of bromine (J. Am. Chem. Soc.). Neither this subsequent treatment nor the reduction in conductivity is acceptable in the context of an application. The reaction mechanism given in J. Am. Chem. Soc. is oxidative polymerisation of 2,5-dihalogen-3,4-ethylene dioxythiophenes.
According to a method disclosed by Baik et al., the 2,5-dihalogen-3,4-ethylene dioxythiophenes are reacted in solution with acids to produce conductive poly-(2,5-dihalogen-3,4-ethylene dioxythiophene)s (W.-P. Baik, Y.-S. Kim, J.-H. Park and S.-G. Jung, Myongji Univ. Seoul, US Patent Appl. 2004/0171790). However, this process does not succeed in overcoming the fundamental problem of a very high halogen excess remaining in the conductive polymer, e.g. as a Br3−-counterion which is associated with the disadvantages described above of stability and the risk of the splitting-off of free halogen.
However, the good conductivity of the poly(3,4-ethylene dioxythiophene)s produced from 2,5-dihalogen-3,4-ethylene dioxythiophene that can be achieved by both processes and the advantageous omission of additional oxidising agents characteristic of both processes, make it desirable to seek alternatives. These alternatives should deliver highly-conductive materials using 2,5-dihalogen-3,4-ethylene dioxythiophenes, without the encumbrance of the abovementioned disadvantages.